Removal of plaque deposits from blood vessels by laser ablation has been widely studied. Typically, a laser catheter is advanced through a blood vessel to a region that is partially or totally occluded by atherosclerotic tissue. One or more optical fibers extend through the catheter to its distal end. Laser energy sufficient to vaporize the atherosclerotic tissue is transmitted through the optical fibers to the obstruction.
One of the problems associated with laser ablation of atherosclerotic tissue is that of ensuring that the laser energy is directed at atherosclerotic tissue rather than normal arterial tissue. It is highly desirable to avoid damage to normal arterial tissue and, in particular, to avoid perforation of the vessel wall.
Laser induced fluorescence spectroscopy has been demonstrated as a technique for distinguishing between normal tissue and atherosclerotic tissue at the distal end of a laser catheter. Low level laser radiation, typically ultraviolet laser radiation, is transmitted in an antegrade direction through an optical fiber in the laser catheter to the occluded region and causes the tissue to fluoresce. The fluorescence is carried in a retrograde direction through the optical fiber to spectrum analysis equipment. By analyzing the fluorescence spectrum, normal tissue and atherosclerotic tissue can be distinguished. When atherosclerotic tissue is identified at the distal end of the laser catheter, high power laser energy is delivered, causing the atherosclerotic tissue to be ablated. Spectral analysis followed by ablation is repeated until atherosclerotic tissue is no longer indicated. This technique is described in U.S. Pat. No. 4,785,806 issued Nov. 22, 1988 to Deckelbaum. Techniques for laser induced fluorescence analysis are also disclosed in U.S. Pat. No. 4,718,417 issued Jan. 12, 1988 to Kittrell et al and U.S. Pat. No. 5,115,137 issued May 19, 1992 to Andersson-Engels et al.
It has been found that tissue fluorescence is strongly attenuated by blood, as described by L. I. Deckelbaum et al in "In-Vivo Fluorescence Spectroscopy of Normal and Atherosclerotic Arteries", SPIE, Vol. 906, Optical Fibers in Medicine III, 1988, pages 314-319, To avoid unacceptable attenuation of fluorescence, the distal end of the optical fiber can be positioned within about 100 micrometers of the tissue to be diagnosed. When laser angioplasty is performed in the presence of blood, it has been difficult to determine the position of the optical fiber tip relative to the tissue. Alternatively, a fluid, such as saline, that is transparent to the laser radiation can be introduced into the treatment region. When laser angioplasty is performed with saline flushing, it has been difficult to determine whether the flushing has effectively removed blood from the region of interest.
It is a general object of the present invention to provide improved methods for laser angioplasty.
It is another object of the present invention to provide methods for controlling the position of an eccentric laser catheter using laser induced fluorescence intensity feedback.
It is yet another object of the present invention to provide methods for determining, by laser induced fluorescence intensity feedback, when the distal end of a laser catheter has crossed an occlusion.
It is still another object of the present invention to provide methods for determining, by laser induced fluorescence intensity feedback, the effectiveness of fluid flushing in a blood vessel.
It is a further object of the present invention to provide a method for determining, by laser induced fluorescence intensity feedback, the position of subsets of optical fibers in a laser catheter relative to tissue.